In a Wavelength Division Multiplexing (WDM) communication network, a network configuration is being changed from a point-to-point network to a ring network using Reconfigurable Optical Add-drop Multiplexer (ROADM) nodes in a backbone line system and a metro core system.
A fixed wavelength signal is assigned for add/drop at each ROADM node, and it has become technically indispensable to remotely change the number of wavelengths at trouble time, or at the time of increasing/decreasing a channel.
In this case, the number of input signals is changed. Accordingly, for an erbium-doped fiber amplifier (Er-Doped Fiber Amplifier: EDFA) disposed in an ROADM, it is requested to perform optical-amplification-gain constant control (Automatic Gain Control: AGC) such that each signal level will not be changed even if the number of signals is changed, which is different from optical-output constant control (Automatic Level Control: ALC) normally used in a system not using an ROADM.
For AGC-EDFAs performing this control, amplifiers following input variations of a few milliseconds have been already in practical use because the speed of a change in the number of wavelengths is a few milliseconds.
In a next-generation ring network, it is requested to effectively utilize wavelength resources in order to meet an increase in traffic. Thus, for an ROADM, it is necessary to employ a dynamic ROADM which is capable of dynamically changing the number of wavelengths in response to transitional variations of the communication capacity. In this case, for an AGC-EDFA, it is necessary to have a response of about 10 microseconds.
To date, there has been a report on a response of a few hundred microseconds. However, in order to achieve a higher-speed ROADM, it is requested to increase the speed further.
Also, further, in an Optical Burst Switching (OBS) system dynamically changing paths of a burst signal, it becomes necessary to have an AGC-EDFA having a response of 10 microseconds or less.
In this manner, there have been increasing demands for a higher speed of 10 microseconds or less, and thus an AGC-EDFA has become a key device in a next-generation network.
For a configuration and control of an AGC-EDFA, it is thought that there are (1) Feedback (FB) control of EDF, (2) Feed-forward (FF) control of EDF (Japanese Patent No. 3811630), and (3) Combination control of FF control and FB control.
Also, for a configuration of an EDFA, there is an example of a configuration combined with a variable optical attenuator (Variable Optical Attenuator: VOA) (Japanese Patent Application Laid-open No. 7-212315).    Patent Document 1: Japanese Patent No. 3811630    Patent Document 2: Japanese Patent Application Laid-open No. 7-212315